Signal transmission system



'Feb. 2, 194 3. v J, 5, N RD I 2,310,198

SIGNAL Tamsuissmu s s'rnus F110 Jan. 17, 1941 wan AND AHPLIFIER lnventofz John E.M'aynard,

His Attorney.

Patented Feb. 2, 1943 Two1. u'ri31 STATE sIGNAL TRANSMISSION SYSTEM John E. Maynard, Schenectady, N. Y., assignor to V General Electric Company, a corporation of New York Application January 1 My invention relates to signal transmission 7, 1941, Serial No. 374,905 61Claims. (or 179-111) systems, and more particularly to improved means for automatically compensating for the effects of varying attenuation produced by such.

systems at different frequencies to be transmitted.

It frequentlyoccurs, for example, that it is desired to transmit signals, represented by currents having frequencies extending over a broad'range, from an amplifier through an impedance network, such as a transmission line, to a load device located at a distant point and where the impedance network, or transmission line, has, different attenuation at the different frequencies to be transmitted and thus undesirably affects the output signals. v

' An object of my invention is to, provide improved compensating means to compensate for such varying attenuation at the' different frequencies thereby to permit faithful reproduction atthe output device of the desired'signals. g

A further object of my invention is to utilize be novel are set forth with particularityin the appended claims. My invention:itself,-however, both as to its organization and method of operation, together with further objects and advantages-thereof may best beunderstood by reference to-thefollowing description'taken in connection with the accompanying drawing inwhlch Fig. 1 illustrates an embodiment of my invention; and Figs, 2 and 3 illustrate'difierent modifications thereof. I V f vIn Fig. l of the drawing, I'liave illustrated a radio receiver which may comprise an amplifier Illtuned to a carrier wave of frequency to be received, which amplifier bears the legend Tuner and amplifier), This amplifier receives a radio wave from an antenna I I and-transmits the wave through a transformer I 2 tea diode rectifier l3. A load resistance I4 and a condenser-l5 in shunt thereto are connected in series with the rectifier l3 and the transformer 12. One end of the resistance I4 is grounded, and signal voltages appear thereacross due to the action of the rectifier the impedance of such a transmission line to con trol the amplification of the amplifier from which signals are supplied to. the line in such a way that the amplification of the amplifier is greater at'the frequencies" at which the transmission line has most' attenuation, thereby to compensate for the effectof such attenuation.

A further object of my invention is to utilize such impedance tocontrol, the: degeneration of the amplifier to effect such compensation.

One application in which my invention finds particular-utility is in connection with aircraft radio receivers, for example, in which it is desired manually to control the output volume of the re ceiver'from a point remote from the receiver in which detection and amplification of thesignals occurs. In accordance with my invention,

such signals may be'transmitted through a transmission line from'an amplifier in the receiver to-a l3. The signal voltages appearing across the resistance it may be audio, voltages, video volt,- ages, or the like. These signal voltages across the resistance M are transmitted through an electron discharge device IS in a, way to hem:-

'plained hereinafter to an electron discharge amplifier device IT, which furthe'r transmitsthe signals throughan amplifier lBto a load devic such, for example, as headphones I9.

It is frequently desirable .to control the transmission of signal voltages between the rectifier l3 and the device I! from a remote position, per- I haps 50 to 100 feet away. In many situations wide range of signal strength encounteredby the antenna II makes it impractical to. control the grid bias potential of an amplifier discharge device in the receiver, since. this potential would have to be varied over such wide limits, as to introduce serious distortionunder certain condiload resistance located at the point'wher'e manual v volume control is to be effected. Signals may then be supplied from a variable portion of this resistance to the loud-speaker, or other signal reproducer, either through the transmission line or otherwise, as desired. r

An'object of my invention ji's to, utilize the impedance of such a transmission line to control the degeneration 'ofthe amplifier, thereby to vary the amplification of the amplifier properly to compensate forthe effects of the varying attenuation of the line with respect, to frequency upon,

the fidelity of the reproduced'signal.

The features of my invention which I believ'eto tions of adjustment.- To avoid such distortion it is preferable tocontrol directly through the use of a voltage dividing resistance or the like the amount of alternating signal. voltage transmitted.

through a shielded two-conductor cable 2! at the receiver. as follows: The shield'covering of the cable" 2| is connectedto ground near the device and is coupled through coupling condenser 26 to To accomplish this end a remote volume control resistance, or voltage divider, 20 is connected the resistance l4. The cathode 21 of the device It is connected through a conductor 28 of the cable 2| to one end of the voltage divider 20, the other end of which is connected to the shield of the cable 2|. The other conductor 29 of the cable 2| is connected to an adjustable tap 30 of the remot volume control resistance 20, and at the other end of the cable, it is connected through a coupling condenser}! to the control electrode 22 of the device H. The cathode 22 of the device I1 is connected through ables resistance 34 to ground, the resistance. being by-passed by a condenser 35 which presents low impedance" to signal current. The control electrode 22 is connected through a resistance to ground. U V

In operation signal voltage across the resistance l4 produces corresponding voltage variation on the control electrode 24. rent of the device It flows through the resistance 2' so that signal voltage appears across the remote volume control 20. A certain portion, as desired, of this signal voltage on the resistance 20 is taken on by the tap 30 and applied across the resistance 36 through the coupling condenser 31. The voltage 'across the resistance 38 isrepeated through the amplifier device l1 and through the amplifier l8 tothe headphones l9.

The device illustrated in this figure is capabl of controlling signal voltage on the resistance l4 over a considerable range of intensities. In a typical example the bias voltage'deveioped by the voltage drop through the resistance was about 6 vo1ts,'and the'device IQ is, therefore, capable of transmitting any signal whose peak voltage developed across the resistor 25 does not exceed 6 volts. f

In-Fig. 2' there is' illustrated'a circuit whichis adapted to the control ofa'larger range of input voltages. In this circuit many of the elementsare similar to those illustrated in Fig. 1 and are given like reference characters. "The cathode 21 of the device I6 is connected through a bias resistance 21 to the conductor". A condenser may be connected in shunt to resistance 21 for by-passlng signal current therearound. The use of condenser 38 results in a slight increase of voltage It is also to be preferred in most situations that the by-pass condenser 28 be omitted. The reason for this preference is that if the by-pass condenser 38 be used, a fixed negative bias is maintained between the control electrode 24 and cathode 21, and with increasing input voltage plate current cut oil is reached at a smaller input volt-' age than if the condenser is omitted. However, a greater proportion of the output voltage is available across the potentiometer ro l: the condenser is used. The necessities of the particular situation should dictate whether the condenser 38 should be used or not.

Although there is substantial capacity between the shield of the cable 2| and the conductors 2| and 29, which may be in some cases as high as The space cur- I 2500 micromicrofarads for 100 foot cable, it has been foundthat suchcapacity does not substantially aflect the transmission of high frequency components of signals from the resistance l4 to thedevice I]. The reason that the apparatus transmits signals from the resistance 14 to the device [1 with substantially equal attenuation at all frequencies in spite of the substantial capacity inthe cable 2| rises from thefact that thedischarge device I6 is connected to act as a degenerative amplifier.

The device IG actsas a degenerative amplifier because the resistance 2|! having one terminal connected to the cathode 21,'is in a common portion of both the input and the output circuits of the device It. The input circuit of the device It may -betraced 7 from the control electrode 24 through the resistance 25, the shield of the cable 21, resistance 20, conductor '28, and to the cathode :1. The output circuit of the device ll may be.

21, so thatthe device I transmits more current transfer through the device l6, as will be eir-v from the ,sourcef23 through the anode 22 and cathode 21 and through the resistance 20. The anfodeg22 is at a flxed positille-potential with respect to ground, and upon increased current flow throughthe resistance 2ll,fthe cathode 21 positive with respect to ground, the rise in voltresistance 20 may,;f0r example, be live times as large as the resistance 21'. If the bias potential developed across theresistan'ce 21 is 6 volts, and

- the potential'across'he resistance 20 is 30-volts,

voltages at the output of the detector. lImuch higher than the bias voltage which issuitable for a discharge device. I a

age of the control electrode 24 is in part offset,

so that only a portion of the voltage across the resistance 25 appears between the control elec trode 24 .and the, cathode 21." A wave impressed across the resistance 2lis thus partially opposed by a. wave consequently appearing across the rein a .well lmown manner, and is termed a degenerative amplifier. The operation of thi amplii'ler including the discharge device il may be more 'fully explained with reference to Fig. 4.

, In' Fig; 4 a signal source It is illustrated as coupled through a coupling condenser 26 to the control electrode 24 of the device ll. Other elements of the. circuit are identical with those illustrated in Fig. 2, th condenser 38 being omitother words, the load impedance may vary (as,

the cathode 21 is designated as e and the instantaneous voltage across the resistances 20 and I1 is designated as er. The amplification factor of the o I TFfl 1 Considering the instantaneous voltages cs. Bi,

and er:as-vectorial-quantities, the following relation exists therebetween:

Eliminating e; from these two equations,- the iol-' lowing equation results:

(.JL. e,.= -j -e 3) Z 11, 1+ i This equation may be restated in terms of es,

because: I

er=Xeo V Equation 5, upon comparison with thetorm of Equation 1, shows that the same result might be secured if the device i8 were not connected degeneratively but instead were constructed to have an amplification iactor P1 1 +p and whose plate resistance The equation for the newly constructed device ii would appear:

I! the amplification factor a of the device l6 be large, for example, of the order oi 10 or or. more, the apparent plate resistance R will bea small fraction of. the actual plateresistance R; of the device i8. e

1 The impedanceiz ,.rwhich is constituted by the entire-load on the. device'ltincluding resistance- 31,-re'sistance 'and1the" load connected across theresistance 20, as well as all: capacities eilectively'in shunt-to the loadand to the resistances,

can easily be made very large with respect to this apparent resistance Rpi of the device i6. When the device it operates as a degenerativeamplifier, has an'apparent plate resistance R which is a small fraction of its actual plate resistance Rp,

and the impedance Z0 is made large with respect to the'apparent plate resistance Rm, the'dev'ice ll tends to produce a constant voltage acrossthe impedance Z0 when a predetermined voltage of any frequency is impressed on the input circuit of the device It from the source 50. Stated in for example, upon a change in frequency), but the voltage developed across the load impedance tends always'to remain constant for a given input voltage of any frequency.

Viewed in still another way, a voltage or impressed on the device It produces a voltage es at the output, The voltage 80 is made considerably smaller than the voltage. e1 by the use oi a very large amount oidegeneration. Now, it the frequency of the-voltage ei impressed on the input of the device, It be increased, the voltage across. the impedance Z is smaller because of the capacity component of Zn which makes the impedance Z0 smaller at the higher frequency. However, with a smaller voltage across the impedance Z0, there is less degeneration and consequently more amplification through the device l6, so that f the voltage as is maintained substantially constant.

In a particular 128R! tube whose amplification factor is 16 and whose plate resistance is 8500 ohms. The resistance 20 is 20,000 ohms and the resistance 31 is 2000 ohms. The source 23 supplies 140 volts between the anode 22 and ground and 35 volts appear across the resistance 20, while 3 /2-volts appear across the resistance 31. This circuit can handle an input signal whose peak voltage across the resistances 20 and 31 reaches 38% volts, and. whose peak voltage between the control electrode 24 and ground reaches 41 volts. Very little change in output is detectable even when a. capac*- ity of 3000 micromicroiarads is present between the conductor 29 and ground, at normal audio frequencies. I

. 26 directly across the resistance 25. The resist-' In Fig. 3 there is illustrated a diflerent modification in which degeneration is produced in a somewhat diiierent way.. Many of the. circuit elements are similar to those illustrated in Figs. 1 and 2 and are given like reference characters. In this circuit the signal voltage across the resistance I4 is coupled through coupling condenser ance 25 is connectedbetween the control electrode 24 and ground, and the cathode 21 is con--' nected to ground through the resistance 31, by passed by the condenser 38 for signal currents. Thea'node 22 is coupled through a condenser 40 tothe control electrode 24 to provide suflicient degeneration to reduce the amplification of the device It to an amount less than unity. The

' anode 22 is further connected through the conductor 28 and the volume control resistor 20 to the positive terminal of the source 23, whose negative terminal is connected to the shield of the cable 2|. .Other connections areas illustratedin Figs. 1 and 2. V

In operation this circuit behaves similarly to t the circuit illustrated in Fig. 2 and is capable of handling about the same amount ofinput signal. Its theory of operation is much the same. In

this circuit as in the circuits of Figs. 1 and 2 the volume control .resistor 20 should bej'large with respect to the apparent plate resistance of the device it. The shunting effect of capacity on high frequency portions of the signal is a minimum.

My inventionis not limited in usefulness to a 0 radio receiver but it may be utilized to provide remote. control oi. any signal, such, for example, as from a public address system, broadcast studio audio channel, or a television transmitter. Furthermore, the attenuatedsignal across the voltage divider 20 may be transmitted elsewhere than through the conductor 29 back to a point near the signal source. It may, for example, be utilized at a point near the divider 20. In any case the device 16 operates to transmit signals at anyfrequency in proper intensity, regardless of large amounts of capacity.

The device may be utilized to good advantage wherever a load devicehas an impedance characteristic such that signals transmitted to it are attenuated differently at different frequencies.

The conductors 28, 29, for example, might be re-' tion at low frequencies and transmission of higher low frequency voltages through device It. In any case, the device is useful for equalizing the attenuation of'a signal having frequencies extending over a band, where the signal must be impressed on a load device having substantial reactance. 7

While I have shown aparticular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since dif-- ferent modifications may be made bothin the circuit arrangement and the instrumentalities employed, and I aim by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the.United States, is:

1. In'combination, a source of signals having frequencies extending over a band of-frequencies, a network through which the signals'from said sourcejare to be transmitted, said network having reactance varying with frequency whereby signals transmitted through said network are attenuated difierently at different frequencies, an electron discharge device having a cathode, an anode, and ajcontrol electrode, an output circuit connected betweensaidjanode and cathode and including said anode and-control electrode for producing in said input circuit a degenerative voltage derived from the signa1 voltage across the impedance of said line, said capacity being of sufllcient magnitude to produce said degenerative voltage in an amount suflicient to reduce the amplification of said amplifier to value less than unity thereby reducing distortion of said signals at said load device caused by the varying impedance characteristics of said line. I

3. A source of signals extending over band of. frequencies, a voltage divider distant from said condenser being sufllciently large to produce degeneration through said device to reduce the amplification therethrough to an amount less than unity, whereby the effect of capacity between said conductors is reduced and the various frequency components of said,signals at said dividertend to be equalized in intensity.

4. A source of signals extending over a band of frequencies, a voltage divider distant from said said network, an input circuit connected between said control electrode and cathode and including said source of signals, and means comprising a capacity between said anode and control electrode for producing in said input circuit a degenerative voltage derived from the signal voltage across said input reactance, said capacity having such value as to cooperate with said reactance in producing sufficient degeneration at any particular frequency in'said'band to reduce the amplification through said discharge device to a value less than unity and to compensate the variation in attenuation of said signals over said band of frequencies caused by said reactance. V

I 2.. An electron discharge amplifier having an anodefa' cathode, and acontrol electrode, a load device remotefrom said amplifier, a transmission line extendiiig' 'irom said amplifier to said load device, a' source; of signals to be transmitted through said to said load device, an input circuit connected between said control electrode and cathode through said source, an output cirsource, connections from said source to said divider comprising a plurality of conductors in close proximity having appreciable capacity therebetween, and means for transmitting a large proportion of the higher frequency components of said signals with respect to the lower frequency components, said means comprising said conductors and said divider, an electron discharge device having a cathode, an anode, and a control electrode included in said connections near said source, and a condenser connected between said anode and control electrode, said con-.

denser being of sumcient capacity to produce degenerative feedback reducing the amplification through said device to an amount less than unity, whereby the various frequency components of said signals at said load device tend to be equalized.

cuit including said 'load device and connected I between said anode and cathode through said transmission line, the impedance of said transmission line being variable with frequency whereby said signals are distorted during transmission through said line, and means comprising the impedance of said line and a capacity between 5. In combination, an'electron discharge de- I vice having an anode, a cathode, and a control electrode, an output circuit connected between said anode and cathode, an input circuit connected between said control electrode and cathode, said input circuit including a source of signals having frequencies extending over a wide range to be transmitted by said device'to said output circuit, a resistance and a source of discharge current for said device connected in said output circuit and located at a considerable distance from said device, the conductors of said output circuit extending from said resistance and current source to said device having substantial capacitance therebetween, a condenser connected between said anode and control electrode, and a load device connected across at least a part of said resistance, the voltage across said conductbrs and said resistance being impressed on said control electrode through said condenser in a degenerative sense, said condenser having such a value that the variation in attenuation of said signals over said band of frequencies is compensated by variation of degeneration of said detron discharge amplifier device having an anode,

vice caused by the reactance of said capacitance.

6. In combination, signal apparatus comprising a source of signals and means for utilizing signals from said source, means located at a point remote from said signal apparatus for attenuating said signals in transmission from said source to said utilizing means, said attenuating means being connected to said source and said utilizing means through conductors having sufllcient capacity therebetween to cause said signals in transmission to be undesirably attenuated differently at different frequencies, and means for substantially eliminating said undesirably diflerent attenuation comprising an eleca cathode. and a control electrode included in said conductors near said source, and a condenser connected between said anode and said control electrode, said condenser being 0! sumcient capacity to produce degenerative feedback reducing the amplification from said source through said device and conductors to said utilizing means to an amount less than unity, whereby the various frequency components of said signals at said utilizing means are substantially qualized.

JOHN E. MAYNARD. 

